This invention relates to a drum brake device. More specifically, it relates to a dual mode drum brake device that functions as a leading-trailing (LT) type when the service brake is applied, and as a duo-servo (DS) type when the parking brake is applied.
This type of dual mode drum brake device has been disclosed in Australian Patent Number AU-B1-53 491/79. Briefly, as shown in FIG. 12, a pair of brake shoes b, c are provided on top of a back plate a. An anchor block d is provided between the adjacent ends of each brake shoe. Two coil springs e, f bias the respective brake shoes in the direction of the anchor block. A hydraulic cylinder g is provided between the other adjacent ends of each brake shoe. A return spring h, provided adjacent to the hydraulic cylinder, biases the two brake shoes towards each other. The parking brake lever j is pivoted on one end i of one brake shoe b. An idler lever k is pivoted so as to swing on the other brake shoe c. A first and second rod I, m are provided between the two brake shoes b, c, wherein one end n of the first rod I engages the parking brake lever j, and the other end o engages the idler lever k. One end p of the second rod m engages the one brake shoe b, while the other end q engages both the other brake shoe c and the idler lever k.
The drum brake device as described above needs the following areas of improvement:
When the parking brake is applied, the adjacent ends of each side of both brake shoes move apart to spread apart the whole brake shoe assembly. In other words, the one end of both shoes also separate at once from the anchor block. When the brake drum which is interlocked with the wheels starts to rotate with the device in this state both brake shoes, the rods, and other components all turn in tandem, wherein one or the other brake shoe collides against the anchor block. The noise so generated is disconcerting to the driver. Additionally, since this impact load is applied repeatedly on the anchor block, the strength of the components becomes critical.
The cumulative effect of the tolerances of each component will be such that the idler lever could abut against the second rod or play could be generated. At the very least, play will be generated when the driver steps on the brake pedal. Accordingly, the idler lever could vibrate creating a strange noise when the vehicle is in motion or the foot brake is applied. Again, this noise can be disconcerting to the driver.